Fluid-mechanical drive units of variable speed ratios have been utilized for many application; however in most applications it has been very difficult to obtain a large torque output at very low output speeds. A further objection with most transmissions of this type is their excessive weight to displacement ratio and their inability to operate at high speeds. Further disadvantages usually involve the inability to operate efficiently over extremely wide ranges of speed ratios, particularly in overcoming starting inertia. Furthermore, such transmissions have usually involved the use of frictional bands which enhance contamination of the hydraulic system which is extremely detrimental to the life of such drive units. A further disadvantage has been the inability to provide a straight line input-output shaft in a very small compact housing that has a very high initial torque capability.
U.S. Pat. No. 3,924,490 granted to Ned D. Mills on Dec. 9, 1975 describes a power transmission speed control apparatus utilizing the hydraulic gear pump, a planetary gear set and a positive displacement hydraulic motor. However, such drive unit is rather complicated and was unable to deliver the high torque requirements at the starting or zero output speed to quickly and efficiently overcome the initial inertia of the load.
One of the principal objects of this invention is to provide a variable ratio fluid-mechanical drive unit that is capable of providing a very high torque output at very low speeds with very high efficiencies over an almost infinite speed ratio from zero to the maximum ratio of the planetary gear set.
A further object of this invention provides a very small, high speed fluid-mechanical drive unit that has a high volumetric displacement to weight ratio.
A further object of this invention is to provide a very small variable ratio fluid-mechanical drive unit that have coaxial input and output shafts in a relatively small housing with a very high fluid displacement to weight ratio.
Further objects of this invention will become apparent upon reading the following detailed description of a preferred embodiment.